A study of coherent synchrotron radiation: intensity enhancement of the far-IR spectrum by exciting single bunch instabilities

2017-02-28T23:29:30Z (GMT) by Tan, Yaw-Ren Eugene
The storage ring at the Australian Synchrotron Light Source (ASLS) is an intense source of radiation that is used in applications such as spectroscopy and imaging. The source of the radiation comes from individual electrons that are grouped in bunches and confined in the storage ring. The observed radiation is usually temporally incoherent and the radiation power scales linearly with the number of electrons, N. If all the electrons have a longitudinal distribution of σz, the degree of temporal coherence of the emitted radiation increases and the power of the radiation will scale as N2 for radiated wavelengths, λ, that are longer than 2πσz. Using this property of coherent synchrotron radiation (CSR), the intensity of the observed radiation at wavelengths longer than λ can be increased by a factor of N above that of the incoherent radiation. With typical values of N around 10^9, the potential enhancement of the radiation power is significant. This property of CSR is used to enhance the radiation in the Far-IR (Footnote: Far Infrared.) spectrum between the wavelengths of 0.3 mm (1 THz) and 3 mm (100 GHz). This region of the radiation spectrum is used by the Far-IR beamline at the ASLS for absorption spectroscopy. The increase in the radiation power will benefit the beamline by increasing the signal-to-noise ratio of the beamline's detector, thereby enabling the measurement of spectra for weakly absorbing materials and providing the capacity to measure spectra at longer wavelengths. To generate CSR, the bunch length in the storage ring is shortened by reducing the momentum compaction factor, αc, using the negative dispersion technique. When αc is reduced by a factor of 100, the storage ring becomes sensitive to perturbations introduced by the accelerator subsystems (e.g., RF system, magnet power supplies and stray electromagnetic fields). An investigation of the various accelerator subsystems showed that the strongest perturbation is at the mains AC frequency of 50 Hz and is caused by currents conducted through the metallic elements of the storage ring. These perturbations limit the shortest achievable bunch length to 1 ps at an electron energy of 3 GeV. A bunch length of 1 ps is not short enough to enhance the radiation power at wavelengths less than 0.3 mm (1 THz). A reduction in the electron beam energy is required to achieve shorter bunch lengths. An alternative method for generating CSR is to excite a longitudinal single bunch instability. The instability that modulates the charge density can be utilised to create periodic bursts of CSR. The onset of the instability creates quasi-periodic bursts of CSR. For CSR to be used as a source of IR radiation, the frequency and the intensity of the bursts of radiation must be constant. Any fluctuation in the frequency or intensity reduces the signal-to-noise ratio (SNR) of any measurements using the CSR. In this thesis a method is developed for optimising the SNR using a microwave diode detector. A microwave diode detector with a fast response time of 1 ns is used to characterise the temporal profile of the CSR bursts. The measurements show a temporal profile with growth and damping rates of 20 kHz which far exceed the natural damping rate expected for the storage ring. Moreover, the results show small oscillations that are believed to be the result of the filamentation of the electron bunch. The observed features of the growth rate, decay rate and small oscillations closely resemble the behaviour seen in numerical simulations by Venturini and Warnock, which show a CSR driven microbunching instability. This model provides a framework for describing the process leading to bursts of CSR. The microwave diode detector is also used to measure the temporal profile's dependence on N. The data collected was used to calculate the change in the SNR to find the optimal working current where the SNR is maximised. The results show that the SNR depends on the burst frequency; in particular when a harmonic of the burst frequency is the same as the characteristic longitudinal oscillation frequency (synchrotron frequency). Measurements were also conducted on the beamline, and the results agree with the observations made using the diode detector. These observations were used to define a low alpha lattice configuration to generate CSR. The CSR created from this configuration was used to measure the absorption spectrum of a sample of N2O with a spectral resolution of 0.025 cm(-1) (750 MHz). For these measurements a Bruker IFS125HR spectrometer was utilised on the IR beamline. This is a challenging measurement as the scan takes 15 minutes to complete, during which the intensity of the source must remain constant. Any fluctuation in the intensity will introduce noise into the spectrum. The results of 20 scans show good agreement between our measurements and previously documented absorption lines for N2O. The outcome shows that the quasi-periodic bursts of CSR can be used to extend the utility of the IR beamline into the Far-IR spectrum. In summary we have used a diode detector to characterise the burst of CSR at the onset of the microwave instability. We have shown that the measurements with a microwave diode detector are in agreement with numerical simulations carried out for this form of instability. A method has also been developed to create a low alpha operational mode that generates bursts of CSR optimised for the IR beamline.